In the drilling of deep wells the drilling bit may be driven by a positive-displacement type motor located down the hole towards the lower end of a drilling string or casing composed of a number of sections through which liquid mud is fed under pressure to drive the motor, scavenge the hole around the bit and carry away cuttings and the like upwardly to the surface through the annular space between the drilling string and the surrounding wall of the hole.
In order to generate the necessary torque in the motor to drive the drilling bit, high liquid mud pressures are required and the motor is exposed to onerous operating conditions by virtue of such pressures and the nature of the driving liquid.
One particularly suitable type of positive displacement motor comprises a rotor rotatable in a housing with the annular space between the rotor and the housing divided into at least two chambers by longitudinally extending separator strips secured to the wall of the housing with a plurality of flexible blades of considerable length attached to the rotor so that they swing out and engage the housing wall in fluid sealing relationship under the pressure of the liquid mud admitted to the chambers and fold inwardly when they engage the separator strips. The number of blades corresponds to or is a multiple of the number of chambers and as they pass a separator strip they are exposed to an inlet for the liquid mud and as they reach a separator strip the liquid mud escape through an outlet, thus relieving the pressure on the blades and allowing them to fold inwardly.
Examples of such motors are disclosed in my U.S. Pat. Nos. 2,852,230 dated Sept. 16, 1956; 3,076,514 dated Feb. 5, 1963 and 3,594,106 dated July 20, 1971. Such motors disclosed in the prior art referred to are undoubtedly useful in the down-hole drilling of wells but it will be appreciated that the blades are exposed to extremely high liquid mud pressures in order to generate the necessary driving torque. Because of such high fluid pressures and the nature of the driving fluid, the motors operate under extremely onerous conditions and any improvement serving to inrease the life of the motor under field operating conditions is important.
The motors disclosed in the prior art referred to above are driven by fluid pressure which enters the motor through inlet ports upstream of fluid flow and leaves the motor through outlet ports downstream of the fluid flow. The fluid pressure to which the flexible blades are exposed is of the order of 400 p.s.i. or greater and the blades are of a length of 20 inches or more. Very high torques are generated in the rotor to drive the drilling bit and as there is a pressure drop across the motor between the fluid inlet and fluid outlet, the turning moment applied to the rotor blades and the rotor is greater at the upstream end of the rotor where the inlet ports are situated than it is at the downstream end of the rotor where the outlet ports are situated. Also it is inherent in the structure that it is the downstream end of the rotor that is exposed to the back reaction torque from the drilling bit that is being driven by the rotor.
Furthermore, as the driving fluid enters the motor at the upstream end and leaves towards the downstream end the fluid, in addition to rotating with the blades about the axis, must also move from the upstream end of the motor to the downstream end thereof thus dissipating some of its energy as such movement does not contribute to driving the blades.